This invention relates to semi-solid molding method and apparatus of the type disclosed in U.S. Pat. No. 6,808,004 and No. 6,901,991 which issued to the assignee of the present invention and the disclosures of which are herein incorporated by reference. The vertical die cast press and the method of using the press for semi-solid molding of metal alloys, as disclosed in these patents, utilizes a shallow shot chamber defined by a water cooled sleeve and a water cooled piston moveable vertically within the sleeve. The '991 patent also discloses the use of an extendable and retractable cooling pin within a central portion of the shot piston which is actuated or moved on its vertical axis by a hydraulic cylinder.
When the vertical die cast press is used fordeveloping a semi-solid slurry within the shot chamber, it is desirable for the slurry to have a uniform consistency or percentage of solids throughout the shot chamber before the slurry is injected or transferred upwardly into the cavity defined by the die set supported above the shot chamber. It is also desirable to minimize the time required to develop the uniform semi-solid slurry within the shot chamber in order to minimize the total cycle time required for molding the slurry into a high strength metal part such as an aluminum vehicle wheel.
As the molten metal within the shot chamber is cooled by the water cooled shot sleeve and the water cooled piston, the more solidified portion of the slurry adjacent the inner surface of the cooling shot sleeve produces a liner or “can” of more solidified metal adjacent the shot sleeve. As disclosed in the above- mentioned patents, this can is trapped by an annular entrapment recess above the shot chamber and opposing the can so that the can collapses and is prevented from flowing inwardly into the more liquid semi-solid metal as it is transferred into the die cavity. However, it has been found desirable to circulate at least some of the cooled pre-solidified metal adjacent the shot sleeve wall and adjacent the top surface of the piston with the more liquified metal so that the uniform slurry is produced more quickly and a greater percentage of the semi-solid metal within the shot chamber is transferred from the shot chamber into the die cavity. A thinner metal can also requires a lesser force to move the shot piston upwardly since there is less pre-solidified can metal to crush into the entrapment recess.